1. Field of the Invention
Embodiments disclosed herein generally relate to the structure of a vertical recording head used in a magnetic disk device.
2. Description of the Related Art
Microwave-assisted magnetic recording (MAMR) has been studied in recent years as a recording method for improving surface density. In MAMR, exerting a magnetic field by a main pole applies an AC field from a spin-torque oscillator (STO) to a medium. Applying an AC field to a medium reduces the coercivity of the medium, which facilitates high-quality recording. Therefore, an important issue for MAMR is to develop an STO which generates a sufficiently large AC field.
With the STO structure 100 shown in FIG. 1, the STO 100 comprises a field generation layer (FGL) 102 for generating an AC field, a spacer 104, and a spin polarization layer (SPL) 106 for transmitting spin-polarized torque. The STO 100 is disposed between the trailing shield 108 and main pole 110 with a cap layer 112 and underlayer 114 present as well. A material having strong vertical anisotropy energy is used for the SPL 106. The STO 100 is also charged by a current from the SPL 106 toward the FGL 102. During this charging, a spin torque oriented in the same direction as the magnetization of the FGL 102 acts on the magnetization of the SPL 106, and a spin torque oriented in the antiparallel direction to the magnetization of the SPL 106 acts on the magnetization of the FGL 102. Because a perpendicular field is applied to the STO 100, the magnetization of the SPL 106 is stable vertically. The magnetization of the FGL 102, however, oscillates in a state having a large in-plane component. Oscillation of the STO 100 in this structure is called T-mode oscillation because the SPL 106 and the FGL 102 oscillate in a T-shape.
A different STO structure 200 is shown in FIG. 2 where the STO 200 comprises an FGL 102 for generating an AC field, a spacer 104, and an SPL 106 for transmitting a spin-polarized torque. The STO 200 is disposed between the trailing shield 108 and main pole 110 with a cap layer 112 and underlayer 114 present as well. The points of difference from FIGS. 1 and 2 otherwise are that the magnetization of the SPL 106 is effectively oriented in the in-plane direction of the film, and both the FGL 102 and the SPL 106 oscillate. Specifically, a current is charged from the FGL 102 toward the SPL 106, and a structure is used in which the SPL 106 has a thin film thickness and a vertical anisotropy field of about several kOe such that the anisotropy field of the SPL 106 is effectively zero. Because the SPL 106 receives a spin torque in the antiparallel direction to the FGL 102 and the FGL 102 receives a spin torque in the parallel direction to the SPL 106 when a current is charged from the FGL 102 to the SPL 106 in this structure, the SPL 106 and the FGL 102 readily oscillate together in-plane, which can generate a high AC field. This structure has the useful feature for high-speed transfer recording that the FGL 102 inverts quickly because inversion of the magnetization of the SPL 106 is not delayed by switching the polarity of the write head field. Oscillation of the STO 200 in this structure is called AF-mode oscillation because the SPL 106 and the FGL 102 oscillate while maintaining an antiparallel state.
The most important feature demanded of an STO is to generate a high AC field. For this purpose, increasing the spin torque acting on the FGL is effective. Since the size of the spin torque is inversely proportional to the density of the current to the STO, increasing the application current obtains higher AC field strength. Too high a charging current, however, increases the temperature of the STO, which increases the probability of failure. Therefore, there is a demand for development of an STO film capable of generating a high AC field by as low a current as possible.